Conventional coal-burning boiler plants and industrial furnaces combust the coal in a reaction zone directly within the furnace, wherein combustion temperatures are high enough to keep slag above its fusion temperature. They are normally operated at an overall stoichiometry greater than 1, which results in generation of substantial quantities of the oxides of nitrogen and the oxides of sulfur, as well as relatively high emission of particulates into the atmosphere. Such furnaces have relatively low energy release per unit-volume and count on the use of refractories to protect against slag erosion. They commonly operate at relatively very low power densities, requiring large-volume "fire boxes" for burning out the carbon content of the fuel, collecting the residual slag and extracting energy from the flame.
In recent years, oil prices have increased by about a factor of ten. Many electric-utility boiler plants and industrial furnaces are caught in a cost squeeze. Trona kilns, for example, require vast quantities of thermal energy; operators of such industrial processes have large capital investments in facilities that are not economically viable at current oil and gas prices.
Conversion of these boilers and furnaces, to burn coal rather than oil or gas, would provide very substantial energy-cost savings; and this can often avoid plant closing, abandonment of capital investment and loss of jobs in the community. But, attempting to burn coal in multi-megawatt boilers originally designed and constructed for oil or gas presents several difficulties that have been thought to be insurmountable: Slag and fly-ash from conventional coal burning will coat the water tubes, sharply reducing efficiency; Emission of sulfur oxides (herein SO.sub.x) and/or nitrogeneous oxides (NO.sub.x) is not merely objectionable socially but, under current clean-air regulations, often prohibited in the urban and semi-urban locales where electricity-generating boiler plants are commonly located. Most often, the space available for installation of coal handling and combustion equipment is severely limited. And, boilers originally designed for oil and gas usually have no provision for slag collection and disposal.
Thus, our society has developed a significant social and economic need for a process and apparatus for conversion (retrofit) of pre-existing boilers and furnaces to adapt them to burn coal. Any such system, to be economically, technically and environmentally acceptable should meet the following requirements:
High power density:--ablout 1.0 million Btu/hour per cubic foot of volume in the primary combustion chamber.
Low No.sub.x :--Consistently less than 450 ppmv and, preferably, less than 250 ppmv in the gases emitted into the atmosphere.
Low SO.sub.x :--Substantially lower than heretofore achievable with conventional combustors and, preferably, reduction of the sulfur-compounds content of the stack gases by about 50 to 90 percent.
Removable of Noncombustibles:--Capture, and removal from the gaseous products of combustion, of 70% to 90% of the noncombustible-minerals content of the fuel before the gaseous products are conducted to the end-use furnace or boiler, depending on the requirements of the specific end use.
Carbon Carryover:--Conversion of substantially all carbon to oxides of carbon before the gaseous products pass to the boiler or other heat-utilization equipment.
Durability:--Protection of the walls of the combustor such that deleterious corrosion and/or erosion of the walls is kept within commercially-acceptable limits.
Thermal Efficiency:--Delivery to the end-use equipment of a gaseous-products stream having about 85 to 90 percent of the chemical potential energy of the carbonaceous fuel. Preferably this energy is delivered partly as sensible heat and partly in the form of carbon monoxide and hydrogen contained in the gaseous-products and readily combustible, to completion, in the end-use equipment.
The present invention provides a system meeting the foregoing requirements.
U.S. Pat. No. 4,217,132 to Burge et al, incorporated herein by reference, describes an apparatus for combusting carbonaceous fuels that contain noncombustible mineral constituents, separating such constituents as liquid slag and conveying a stream of hot combustion products to a thermal energy utilization equipment, such as a boiler. In the Burge et al apparatus solid carbonaceous fuel (e.g. powdered coal) is injected into a combustion chamber and, simultaneously, a stream of oxidizer (e.g. preheated air) is introduced tangentially into the chamber to produce high velocity swirling flow conditions therein suitable for centrifugally driving most of the liquid slag to the inside walls of the chamber. The apparatus described in the '132 patent is a first-generation, high-power-density slagging combustor. The present invention relates to improvements in slagging combustors, resulting from extensive study and development including recognition of requirements peculiar to adapting slagging combustors to industrial furnaces and electric-utility boilers originally designed and constructed to use oil and/or natural gas. Our apparatus, described herein, is a slagging combustor, belonging to the same general class as that disclosed by Burge et al. Our apparatus includes several improvements and is, to the best of our knowledge, the only extant technology for simultaneously removing substantially all slag, controlling NO.sub.x and SO.sub.x emissions, and avoiding carryover of unburned carbon and other particulate, while operating at high efficiency, having commercially-acceptable durability and being small enough to be retrofitted into the limited space normally available in commercial-sized industrial and utility plants.